PROTEIN

Proteins are polymers of amino acids that contain the elements carbon, hydrogen, oxygen, nitrogen and sulfur. Particular emphasis is given to the indispensable amino acids, i.e., those that must be in the food supply because they either cannot be synthesized in the body or they cannot be synthesized in adequate quantity to meet the needs for optimal rates of growth in children. The indispensable amino acids are: lysine, methionine, threonine, tryptophan, leucine, isoleucine, valine, phenylalanine, arginine and histidine. A dietary supply of arginine is not required by adults. The amino acids supplied from dietary protein are needed for synthesis of body proteins in muscle, organs, bone and skin, and for synthesis of enzymes, certain hormones, antibodies and a host of specialized products.

Deficiencies: Deficiencies of protein or of one or more of the indispensable amino acids will lead to reduced growth in children or loss of muscle mass in adults. Susceptibility to disease will be increased, and in extreme cases, death may result. Protein-energy malnutrition is often described in its two extreme forms: marasmus which is a generalized wasting due to a deficiency of both protein and energy, and kwashiorkor which is characterized by edema and a deficiency of both protein quantity and quality (i.e., poor pattern of indispensable amino acids).

Food sources: Virtually all unprocessed foods contain protein, and the protein of these foods is characterized by the quantity and ratio of indispensable amino acids (protein quality) contained therein. Animal proteins (meat, milk, eggs) are considered to be excellent in both protein quantity and quality. Plant proteins vary in protein quantity and quality. Members of the bean family: soybeans, kidney beans, navy beans, and lima beans are rich in protein, and the protein quality is quite good as well. Split peas and lentils fall into the same category. Potatoes are rich in protein quality but not in quantity. Breads, cereals, fruits and most leaf and stem vegetables are not considered good sources of high-quality protein. Combining certain foods together (e.g. beans with corn) results in a complementary effect in which the protein quality of the combination is superior to either food consumed alone.

Diet recommendations: Recommended dietary allowances attempt to make allowances for protein quality differences in foods typically consumed by the US population. Surveys have indicated that animal-source proteins contribute 65% of the daily protein ingested by humans in the US. Protein requirements increase with advancing age up to 50 years (63 g/day for males, 50 g/day for females) and then plateau. Children of both sexes between 7 and 10 years of age are estimated to require 28 g/day. Additional protein is needed for pregnancy (+10 g/day) and lactation (+15 g/day). Dietary surveys indicate that adult Americans consume more-than-adequate amounts of protein (men, 90 g; women 70 g).

For further information:

Fuller, M. F. & Garlich, P. J. (1994) Human amino acid requirements: can the controversy be resolved. In: Annual Review of Nutrition (Olson, R. E., Bier, D. M. & McCormick, D. B., eds.), vol. 14, pp. 217-241. Annual Reviews, Inc. Palo Alto, CA.

Emmert, J. L. & Baker, D. H. (1995) Protein quality assessment of soy products. Nutr. Res. 15: 1647-1656.

Prepared By:

David H. Baker, Ph.D.
Professor of Nutrition
Department of Animal Sciences
University of Illinois
1207 W. Gregory Drive
Urbana, IL 61801
Phone: 217-333-0243
FAX: 217-333-7861
Email: d-baker1@uiuc.edu

N. J. Benevenga, Ph.D.
Professor of Meat, Animal Science
and Nutritional Science
University of Wisconsin
1675 Observatory Drive
Madison, WI 53706-1284
Phone: 608-263-4313
FAX: 608-262-5157
Email: njbenev@facstaff.wisc.edu